It has been suggested to use O.sup.6 -alkyl guanine derivatives possessing O.sup.6 -alkylguanine-DNA alkyltransferase depleting activity in order to enhance the effectiveness of chemotherapeutic alkylating agents used for killing tumour cells. There is increasing evidence that in mammalian cells the toxic and mutagenic effects of alkylating agents are to a large extent a consequence of alkylation at the O.sup.6 -position of guanine in DNA. The repair of O.sup.6 -alkylguanine is mediated by ATase, a repair protein that acts on the O.sup.6 -alkylated guanine residues by stoichiometric transfer of the alkyl group to a cysteine residue at the active site of the repair protein in an autoinactivating process. The importance of ATase in protecting cells against the biological effects of alkylating agents has been most clearly demonstrated by the transfer and expression of cloned ATase genes or cDNAs into ATase deficient cells: this confers resistance to a variety of agents, principally those that methylate or chloroethylate DNA. Whilst the mechanism of cell killing by O.sup.6 -methylguanine in ATase deficient cells is not yet clear, killing by O.sup.6 -chloroethylguanine occurs through DNA interstrand crosslink formation to a cytosine residue on the opposite strand via a cyclic ethanoguanine intermediate, a process that is prevented by ATase-mediated chloroethyl group removal or complex formation.
The use of O.sup.6 -methylguanine and O.sup.6 -n-butylguanine for depleting ATase activity has been investigated (Dolan et al., Cancer Res., (1986) 46, pp. 4500; Dolan et al., Cancer Chemother. Pharmacol., (1989) 25, pp 103. O.sup.6 -benzylguanine derivatives have been proposed for depleting ATase activity in order to render ATase expressing cells more susceptible to the cytotoxic effects of chloroethylating agents (Moschel et al., J. Med.Chem., 1992, 35, 4486). U.S. Pat. No. 5,091,430 and International Patent Application No. WO 91/13898 Moschel et al. disclose a method for depleting levels of O.sup.6 -alkylguanine-DNA alkyl-transferase in tumour cells in a host which comprises administering to the host an effective amount of a composition containing O.sup.6 -benzylated guanine derivatives of the following formula: ##STR2## wherein Z is hydrogen, or ##STR3## and R.sup.a is a benzyl group or a substituted benzyl group. A benzyl group may be substituted at the ortho, meta or para position with a substituent group such as halogen, nitro, aryl such as phenyl or substituted phenyl, alkyl of 1-4 carbon atoms, alkoxy of 1-4 carbon atoms, alkenyl of up to 4 carbon atoms, alkynyl of up to 4 carbon atoms, amino, monoalkylamino, dialkylamino, trifluoromethyl, hydroxy, hydroxymethyl, and SO.sub.n R.sup.b wherein n is 0, 1, 2 or 3 and R.sup.b is hydrogen, alkyl of 1-4 carbon atoms or aryl. Mi-Young Chae et al., J.Med.Chem., 1994, 37, 342-347--published after the priority date of the present application--describes tests on O.sup.6 -benzylguanine analogs bearing increasingly bulky substituent groups on the benzene ring or at position 9. Compound No. 6 described therein is O.sup.6 -(2-pyridylmethyl)guanine, which in this application is called O.sup.6 -(2-picolyl) guanine. However in the Results and Discussion at pages 342-343 of the Chae et al. paper, Compound No. 6 is not highlighted as being of interest but is grouped among "remaining compounds" which "exhibited intermediate activity" (Page 343 Lines 12-15 of the text). The authors confirm their earlier observations (J.Med.Chem., 1992, 35 4486) that only allyl or benzyl substituents at the O.sup.6 position of guanine efficiently inactivated ATase (page 343 lines 21-23 of the text).
O.sup.6 -benzylguanine has limitations in its use as an ATase inactivator. It is more stable than would be desirable, resulting in a long survival time in an animal to which it is administered. It has a level of potential toxicity both alone and in combination with chloroethylating agents which is also undesirable and which may be related to the survival time.
The compounds of the present application exhibit different ATase inactivating characteristics from O.sup.6 -benzylguanine and in some cases the activity is up to 8 times greater than that of O.sup.6 -benzylguanine. Different half-life and toxicity characteristics have also been observed. Therefore, it is an object of the present invention to provide novel compounds useful for depleting ATase activity in order to enhance the effects of chemotherapeutic agents such as chloroethylating or methylating anti-tumour agents.
Another object of the invention is to provide pharmaceutical compositions containing compounds which are useful for depleting ATase activity. A further object of the present invention is to provide a method for depleting ATase activity in tumour cells. A still further object of the invention is to provide a method for treating tumour cells in a host.